Because of में. हमारे is oblique case, which is used when there is a post position. में is a post position.
This may help better explain: http://www.learning-hindi.com/post/1133921729/lesson-50-postpositions
I suppose there is nothing is wrong with saying things that way, although that's not how I would say it. I am also from an English speaking family.
BUT, that's not what this particular sentence says. You are supposed to be translating the sentence given. This particular sentence says "Grandmother is in our house". There is no mention of we or of grandmother staying at our house. For all we know, she could just be popping in for a few minutes. You're adding much more than is there.
My point was that this sentence structure does not sound normal to an English speaker. Is Grandmother an honored guest or a hostage? I get it that you are from a 'Hinglish' speaking family, and this does sound normal to you. Now that I've completed the course, I realize that your point was that we need to just accept the sentence as is, and quit trying to match it up to our syntax. That was the greatest challenge of learning Hindi. Trying to re-program my brain to the sentence structure. Now I'm on to Spanish and it is so much easier for me personally, but I really enjoyed learning a little bit of basic Hindi. Thanks to all the moderators for volunteering their time.
The sentence is, "Grandmother is in our house". Not sure why that is difficult. Grandmother could just have came for a visit, but we should never try to add more than what's there.
This is why you should never assume. I am a native English speaker. English is my first language, Spanish is my second language. My family is bilingual, Spanish & English, from Gibraltar. I encourage you to google Llanitos, both the language and the people. It's pretty interesting how the language evolved. This is my family roots.
You have to realize when learning other languages that many things are expressed differently. The challenge of learning a new language is teaching yourself to think in that language and its various nuances to expression.
Spanish and Hindi have a lot of similarities in how things are expressed. For example, in Spanish we would say, "Me gusta ensalada." Directly translated to English would be "Salad is pleasing to me".
In Hindi, this sentence would be expressed in just that same way, "मुझे सलाद पसंद है". Again, directly translated to English would be "Salad is pleasing to me".
I find it helps to make a practice of listening to native speakers to improve in this way. Amazon prime has a lot of great movies and series in Hindi and Spanish, as does youtube.
I can definitely recommend some great Hindi movies!
If you would ever like to team up, I am more than happy to do so. We can help each other in both Hindi and Spanish :). I lost a lot of my Spanish because I didn't use it for over 30 years, so now I'm trying to brush up and awaken that part of my brain.
Now it's embarrassing when I go to visit family abroad. My grandparents who raised me thought they were helping by only speaking to us kids in English, because assimilation was a priority for them, since they came to the US in '67.
I wish you good luck and prosperity on your language learning journey! I find it's important to keep an open mind. Allow your mind to be mallable. :)
Bhole Baba ki kripa! <3